This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The broad goal of the project is to determine age-related changes in the physiology of the hearing end organ (the cochlea) in the common marmoset (Callithrix jacchus) by using non-invasive procedures. The procedure measures sounds generated by the cochlea (otoacoustic emissions or OAEs) in response to acoustic stimulation of the ear. OAEs are presumed to be generated by the activity of outer hair cells which are important for improving hearing performance. With hearing loss, hair cells are damaged or unable to function effectively, and this causes a reduction in OAEs. Thus, OAEs can provide a direct measure of cochlear status, in particular the status of outer hair cells. The immediate goal of the project is to ascertain whether marmosets demonstrate OAEs that are similar to humans and Old World primates, and whether they are affected by aging. The long-term goal is to carry out invasive physiological studies and to develop prosthetic implants for treating hearing loss. Specific Aims The two specific aims of the project are (1) to determine the effects of hearing on normative evoked otoacoustic emissions, and (2) the effect of aging on OAEs when the contralateral ear is stimulated. We proposed to evaluated two types of evoked OAEs: distortion product (DP) and click-evoked (CE) OAEs in four different age groups. Because OAEs are sensitive to the stimulation parameters, we will first determine the most optimal stimulation parameters for eliciting the strongest OAEs, and ascertain whether these parameters are similar to humans and Old World primates.